The Mythical Man-Month

nine women one month

The Mythical Man-Month: Essays on Software Engineering’ is a book on software engineering and project management by Fred Brooks, whose central theme is that ‘adding manpower to a late software project makes it later.’ This idea is known as Brooks’ law, and is presented along with the second-system effect (the tendency of small, elegant, and successful systems to have elephantine, feature-laden monstrosities as their successors) and advocacy of prototyping.

Brooks’ observations are based on his experiences at IBM while managing the development of OS/360. He had mistakenly added more workers to a project falling behind schedule. He also made the mistake of asserting that one project — writing an Algol compiler — would require six months, regardless of the number of workers involved (it required longer). The tendency for managers to repeat such errors in project development led Brooks to quip that his book is called ‘The Bible of Software Engineering,’ because ‘everybody quotes it, some people read it, and a few people go by it.’ The book is widely regarded as a classic on the human elements of software engineering. The work was first published in 1975

Assigning more programmers to a project running behind schedule will make it even later, because of the time required for the new programmers to learn about the project, as well as the increased communication overhead. When N people have to communicate among themselves, as N increases, their output M decreases and can even become negative: i.e., the total work remaining at the end of a day is greater than the total work that had been remaining at the beginning of that day, such as when many bugs are created.

Brooks illustrates the fallacy of adding workers to speed the work by counterexample: If one woman can produce a baby in nine months, then nine women should be able to produce a baby in one month. The reason that this is false is that gestation is a sequential process, whose stages cannot run in parallel. If nine women get pregnant at the same time, in nine months they will produce nine different babies.

The Second-system effect proposes that, when an architect designs a second system, it is the most dangerous system he will ever design, because he will tend to incorporate all of the additions he originated but did not add (due to inherent time constraints) to the first system. Thus, when embarking upon a second system, an engineer should be mindful that he is susceptible to over-engineering it.

The author makes the observation that in a suitably complex system there is a certain irreducible number of errors. Any attempt to fix observed errors tends to result in the introduction of other errors.

Brooks wrote ‘Question: How does a large software project get to be one year late? Answer: One day at a time!’ Incremental slippages on many fronts eventually accumulate to produce a large overall delay. Continued attention to meeting small individual milestones is required at each level of management.

To make a user-friendly system, the system must have conceptual integrity, which can only be achieved by separating architecture from implementation. A single chief architect (or a small number of architects), acting on the user’s behalf, decides what goes in the system and what stays out. The architect or team of architects should develop an idea of what the system should do and make sure this vision is understood by the rest of the team. A novel idea by someone may not be included if it does not fit seamlessly with the overall system design. In fact, to ensure a user-friendly system, a system may deliberately provide fewer features than it is capable of. The point is that if a system is too complicated to use, then many of its features will go unused because no one has the time to learn how to use them.

What the chief architect produces are written specifications for the system in the form of the manual. It should describe the external specifications of the system in detail, i.e., everything that the user sees. The manual should be altered as feedback comes in from the implementation teams and the users.

When designing a new kind of system, a team will design a throw-away system (whether it intends to or not). This system acts as a ‘pilot plant’ that reveals techniques that will subsequently cause a complete redesign of the system. This second, smarter system should be the one delivered to the customer, since delivery of the pilot system would cause nothing but agony to the customer, and possibly ruin the system’s reputation and maybe even the company.

Every project manager should create a small core set of formal documents defining the project objectives, how they are to be achieved, who is going to achieve them, when they are going to be achieved, and how much they are going to cost. These documents may also reveal inconsistencies that are otherwise hard to see.

When estimating project times, it should be remembered that programming products (which can be sold to paying customers) and programming systems are both three times as hard to write as in-house programs. It should be kept in mind how much of the work week will actually be spent on technical issues, as opposed to administrative or other non-technical tasks, such as meetings.

To avoid disaster, all the teams working on a project should remain in contact with each other in as many ways as possible — e-mail, phone, meetings, memos etc. Instead of assuming something, the implementer should instead ask the architects to clarify their intent on a feature he is implementing, before proceeding with an assumption that might very well be completely incorrect. The architects are responsible for formulating a group picture of the project and communicating it to the others.

Much as a surgical team during surgery is led by one surgeon performing the most critical work himself while directing his team to assist with or overtake less critical parts, it seems reasonable to have a ‘good’ programmer develop critical system components while the rest of a team provides what is needed at the right time. Additionally, Brooks muses that ‘good’ programmers are generally five to ten times as productive as mediocre ones.

Software is invisible. Therefore, many things only become apparent once a certain amount of work has been done on a new system, allowing a user to experience it. This experience will yield insights, which will change a user’s needs or the perception of the user’s needs. The system should, therefore, be changed to fulfill the changed requirements of the user. This can only occur up to a certain point, otherwise the system may never be completed. At a certain date, no more changes should be allowed to the system and the code should be frozen. All requests for changes should be delayed until the next version of the system.

Instead of every programmer having his own special set of tools, each team should have a designated tool-maker who may create tools that are highly customized for the job that team is doing, e.g., a code generator tool that creates code based on a specification. In addition, system-wide tools should be built by a common tools team, overseen by the project manager.

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